In recent years, flat-panel display devices have been vigorously developed. By virtue of such advantageous features as light weight, small thickness and low power consumption, special attention has been paid to liquid crystal display devices among others. In particular, in active matrix liquid crystal devices in which thin-film transistors (TFTs) are incorporated in respective pixels as switching elements, there is known such a configuration that a transmissive liquid crystal display panel and a backlight are combined.
In a structure wherein a top-gate-type polysilicon TFT including a polysilicon semiconductor layer is applied as a switching element, a problem arises with an increase of OFF current due to an increase in luminance of a backlight. Specifically, a drain current of a TFT increases by the absorption of backlight in the polysilicon semiconductor layer. The increase in drain current conspicuously occurs in the state in which the TFT is in the OFF state, and such a drain current is called “photo-leakage current”. In recent years, to meet a demand for a higher luminance of the screen, there is a tendency to increase the luminance of backlight. Consequently, there is concern that the display quality is adversely affected by, e.g. crosstalk or flicker due to the increase in photo-leakage current.